impedance control specsBrandon,
First I can tell you that we have gotten our specs down so that there is
absolutely no
change to the material thickness. All the boards tested were accurate and we
began
having board built without the test and had some of those tested here. They
were also
100 within spec if not right on the head.
I'm not "The man" on this issue but I have spent a lot of time with our
specifications.
Our specs (to an extent) area at www.robertsondne.com . They are modular so
that
you can choose between a couple different ones. There is also a calcualtor,
based
and tested on several others including polor. The advantage is that you can
calculate up to 12 layers at once instead of chasing your tail with the 2
layer versions.
(resource.xls) it also allows you to build your stackup and record it along
with other
settings.
First there is the material itself. IPC spec material tolerance is only 1mil
for prepreg
and 2 for core material. (check for yourself) If you do the math any more
will be out of
impedance tolerance. +/-10% for 50Ohm. I provide a spec that the material
may
change the value of the material tolerance (+/-)
Second if possible, sandwich a prepreg between two planes. Use this to
adjust to
whatever thickness remains. You don't want to adjust the dielectric
thicknesses
to account for the board thickness, so this is the best way I have found.
Third, trace tolerance is +/- 1mil. If you do the math, that is all that is
allowed,
so a board house running advanced technology will be required.
Fourth specify the impedance tolerance and the Er value that your
calculations
were based on so the material is matched up. (I have to say that because I
get different stories from fabricators)

Test for the first few you do and find out the "fall out" amount and get
feedback
on what the actual stackup was to verify future values.

Seperate you minimum trace note from your controlled impedance note to
clarify
things. Some manufacturers like to calculate things for themselves. I use 8
mil
traces for controlled impedence and 6 mil min trace. (don't want them to do
calculations for 6mil)


Make very sure (very dam sure) that they call you if they need to make the
thicknesses
different than specified. It's more than a matter of who is right, but
documenting what
was done.


Also there a few combination you will find in the trace/dielectric thickness
8mil trace
on 1 oz copper with 12mil dielectric has been working for me.

There are several different stackups, but find one that works best and stick
with it.
Controlled impedence is normally done with high frequency so at first,
practice
sandwiching your signal layers with plane layers to eliminate crosstalk then
move
on to trace-trace stack ups. (I will stay with plane-trace-plane for a long
time)


I don't take credit for this program because I riped everyone off to make
it.
I would like to thank them all for the cullmination of their information.
I'm working on making this a stand alone program for users of any layout
program.

I hope this helps. If so pass it on.

Chris Robertson
[log in to unmask]

Senior Designer
Lockheed-Martin Services Inc.
4912 Research Dr.
Huntsville, AL 35805
(256) 722-2626






  Good Day!

    We are having a raging debate about specifying controlled impedance on
our fabrication drawings.

    The main part of the debate is over laminate thickness specifications.
I would like to know how other companies are handling these issues.

          Are you specifying the individual laminate thickness?

          Are you specifying the individual laminate thickness tolerance?
If so, how do you handle deviations between different fab houses beyond the
tolerance?

  It seems that every time we use a different fab house, they like to use
their standard laminates.  While this is also a benefit to us, it causes
problems when our boards are microsectioned and the board doesn't agree with
the print.

  We will continue to specify trace width, copper thickness and impedance,
all with tolerances.

  Any input will be appreciated!

          Brandon Luther

          Dataram Corp.

  (609) 799-007  x2310